spi: stm32: fix example with st, spi-midi-ns property

This patch aligns example with the optional property description,
removes status and replace spidev unvalid compatible with
Aarvark SPI Host Adapter one.

In slave mode, Aardvark SPI Host Adapter requires 4ms delay
between the end of byte n and the start of byte n+1, hence the
use of the optional property st,spi-midi-ns.

Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Mark Brown <broonie@kernel.org>